The present invention relates to improved thermoplastic compositions for plenum wire and cable construction which exhibit enhanced flame-resistant and low-smoke properties, as well as refined physical properties. In addition, the invention is directed to the insulated wire and jacketed electrical cables produced utilizing the improved thermoplastic compositions.
Plenum cables are the electrical and/or telecommunication cables (or wires) which are installed in environmental air spaces in the interior of many commercial and residential buildings. Previously, as a result of the use of closed loop heating and ventilation systems and the potential of recirculating the smoke produced by the burning of the cables, metal conduits were required for installation. However, with the advances in the last ten years in material development, particularly in regard to the use of fluorocarbon polymeric materials in plenum cable construction and the employment of a separate "jacketed" layer which surrounds the "insulated" electrical conductors, the National Electrical Code (NEC) permitted in the late 1970's the use of insulated and jacketed cables with fire-resistant and low-smoke properties in environmental air spaces without the use of metal conduits.
Although the use of fluorocarbon polymeric materials, such as fluorinated ethylene propylene, as a means of providing insulated and jacketed plenum cables having low smoke generating characteristics in conjunction with flame retardancy characteristics proved to be beneficial, such fluorocarbon polymeric materials are expensive and contain fluorine. When the fluoroplastic materials are heated to high temperatures, for example, in a fire, they release a complex series of potentially toxic and undesirable gases. Consequently, a number of alternative polymeric materials having attractive physical and insulating characteristics, as well as flame resistant and low smoke properties, such as poly(vinyl chloride), have been utilized in plenum cable construction.
Over the last few years, the present inventors and others have developed a number of poly(vinyl chloride) compositions in order to not only enhance the fire-resistant and low-smoke properties of the separate insulation and jacketing layers, but also to improve the flexibility characteristics of the plenum cables. In this regard, the flexibility of the plenum cables is extremely important for installation purposes in that the plenum cables must be flexible enough so that the installer can feed the cable through plenum areas with ease.
However, in order to improve flexibility, various plasticizers are normally added to the poly(vinyl chloride) compositions which are utilized to insulate and/or jacket the electrical conductors. Unfortunately, these plasticizers also increase the flammability of the poly(vinyl chloride) compositions. While the increase in flammability can be reduced somewhat through the use of various flame retardants, the addition of the flame retardants and other processing additives to the plasticized poly(vinyl chloride) compositions in many instances increases the amount of smoke produced by the burning poly(vinyl chloride) compositions.
As a result, a number of poly(vinyl chloride) compositions have been formulated in an attempt to overcome the flammability and smoke generation produced by the plasticized poly(vinyl chloride). However, improvements in the fire-resistant and low-smoke producing properties of the poly(vinyl chloride) compositions, as well as the cables' overall physical properties, would still be quite beneficial for plenum cable construction.
The present invention is directed to new and improved low cost thermoplastic compositions which may be utilized, in various embodiments, for constructing the insulation and jacketing components of plenum wire. While various other patents, including U.S. Pat. No. 4,670,494, entitled "Flame Retardant Low Smoke Poly(vinyl chloride) Thermoplastic Composition" issued on Jun. 2, 1987 to Nicholas J. Semenza, and assigned to the Gary Chemical Corp., describe poly (vinyl chloride)-chlorinated poly(vinyl chloride) compositions and the problems associated with the high flammability produced by the incorporation of plasticizers necessary for flexibility and the increased smoke generation produced by the addition of various flame retardants, the plenum compounds disclosed therein do not exhibit the enhanced properties produced by the present invention.